Due to the extensive use of fossil fuels, global CO2 emissions have increased steadily over the past several decades, with possibly deleterious consequences on the environment. Therefore, significant efforts have been made to reduce CO2 emissions by applying various capture and separation technologies. Compared to most traditional techniques, such as amine-based solvent absorption, passive membrane CO2 separation technology has drawn considerable attention, since the use of high performance passive membranes could offer a significant reduction in energy cost. Moreover, membrane-based separation is an environmentally benign and sustainable methodology.
In membrane separations, the two major parameters of concern are gas permeability and selectivity. Permeability (PA) is defined as the product of gas flux and membrane thickness divided by the pressure difference across the membrane, which is determined by both the gas solubility (SA) and gas diffusivity (DA) (PA−SA×DA), while the gas selectivity (αAB) is the ratio of permeability coefficients of any two gases
      (                  α        AB            =                        P          A                          P          B                      )    .Given that the target gas pair for flue gas separation is generally CO2/N2, polymer membranes with both good CO2 permeability and CO2/N2 selectivity are desirable. In general, polymer membranes are usually evaluated using the so-called Robeson plot and an empirically derived upper-bound that reflects the inherent trade-off relationship between the permeability and selectivity. In this trade-off, membranes that yield high permeabilities are generally less selective and vice-versa (e.g., L. M. Robeson, Journal of Membrane Science 1991, 62, 165-185; L. M. Robeson, Journal of Membrane Science, 2008, 320, 390-400).
Unfortunately, the gas separation performance of commercially known membranes generally falls significantly below Robeson's upper-bound and does not meet the separation efficiency for practical CO2/N2 separation. Moreover, as gas separation is particularly needed for large volume processing of gas flows emanating from power plants, membranes with exceptionally high permeability are needed for practical reasons. Simply pursuing membranes with enhanced selectivity (e.g. CO2/N2 higher than 30) will not improve separation efficiency due to the limitation of pressure difference in practical separation, and membranes with high selectivity but poor permeability would not be useful in CO2 separation. Therefore, a balance between membranes with high permeability and good selectivity will be required for the separation of CO2 from flue gas mixtures.
As a typical rubbery polymer at room temperature, polydimethylsiloxane (PDMS) is known to possess some of the highest permeabilities to various gases, e.g., the reported permeability is 3800 barrer for CO2, 890 barrer for H2, and 400 barrer for N2 (e.g., T. C. Merkel, et al., J. Polym. Sci. Pol. Phys., 2000, 38, 415-434). PDMS has drawn much attention also due to its good physical and mechanical properties (e.g., ductility), good aging profile, low cost, thermal stability, and ease of processing. For CO2/N2 separation, the reported selectivity of typical PDMS membranes is around 9.5 (Merkel et al. 2000, supra). In PDMS-like rubbery polymer membranes, the liquid-like polymer matrix is known to have poor size-sieving ability (K. Ghosal, et al., Polym. Adv. Technol., 1994, 5, 673-697). If the CO2/N2 selectivity of those PDMS membranes can be improved while maintaining high permeability, the designed membrane would meet the practical target in efficiency and cost.